Ensure that we can build from dpkg_build element workspaces

James Ennisrequested to merge
jennis/fix_building_in_workspaces into master

Previously, we could not build from a dpkg_build element workspace because during the build we would look for the the DEBIAN/control file of a package in the following path: sandbox.get_directory(), %{build-root}, debian, ${package}, DEBIAN, control

When building from a workspace, we need to look into the workspace's debian subdir: %{abs_workspace_path}, debian, ${package}, DEBIAN, control

This patch checks whether a workspace is open for the dpkg_build element and then looks at the correct dir.

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